Well, under US Law, I'd walk.

"Defendants seeking to rely on this defense argue that they should not be held liable for their actions as a crime because their conduct was necessary to prevent some greater harm"

"the defendant must affirmatively show (i.e., introduce some evidence) that (a) the harm he sought to avoid outweighs the danger of the prohibited conduct he is charged with; (b) he had no reasonable alternative; (c) he ceased to engage in the prohibited conduct as soon as the danger passed; and (d) he did not himself create the danger he sought to avoid"

a) 5>1
b) 2 tracks, one cu... I mean, 2 tracks, one train
c) Well, I did stop killing after that
d) Hello. The one tying the people did it.

Apparently no correspondence in English law. Figures.